This invention relates to DC converters, and is particularly concerned with DC converters in which a transformer provides coupling between an input circuit and an output circuit of the converter. DC converters are alternatively referred to as DC-to-DC converters, DC or DC-to-DC regulators, switch mode regulators, converters, or power supplies, etc.; the term xe2x80x9cDC converterxe2x80x9d or simply xe2x80x9cconverterxe2x80x9d is used herein to include all such terms.
These terms as used herein also embrace converters for which a supply coupled to the input circuit can comprise a relatively low frequency alternating or varying voltage optionally with a DC component, and/or for which the output can comprise a relatively low frequency alternating or varying voltage optionally with a DC component (e.g. a telephone ringing signal), in other words a DC converter that can operate in all quadrants.
There is a wide variety of known DC converter topologies which are derived from at least one transformer in combination with components of at least one forward converter (also referred to as a buck regulator) and/or at least one flyback converter (also referred to as a boost regulator).
For example, a so-called single-ended forward converter is known which comprises a buck regulator together with a transformer and a rectifier diode, the transformer having a single primary winding which is connected via a switch of the buck regulator to a DC input. As is well known, in a single-ended forward converter it is necessary to take steps, involving additional components or circuitry, to ensure that the transformer core is reset in each switching cycle, i.e. to ensure that the net magnetization of the transformer core in each cycle is zero, to avoid magnetic saturation of the core.
It is also known to replace diodes in DC converters by switches, e.g. MOSFETs, which are operated synchronously in the converter switching cycles. The synchronous switches can provide lower forward voltage drops and less power dissipation than diodes, resulting in more efficient operation especially in the case of DC converters providing high current and/or low voltage outputs.
In order to further improve efficiency, and to reduce potential radio frequency interference (RFI) and electro-magnetic interference (EMI), it is desirable for switches in DC converters to be operated as much as possible with zero voltage switching (ZVS) or zero current switching (ZCS). To this end it is known to incorporate quasi-resonant circuits in some DC converters, controlling the timing of switch closing and/or opening to achieve ZVS or ZCS.
For example, Jain U.S. Pat. No. 5,159,541 dated Oct. 27, 1992 and entitled xe2x80x9cAsymmetrical Pulse Width Modulated Resonant DC/DC Converterxe2x80x9d describes a DC converter in which a chopper comprising two switches is coupled via a resonant circuit, including a series capacitor, to a primary winding of a transformer, a secondary winding of the transformer being coupled via rectifiers to an output filter circuit. This patent also discusses various known forms of DC converters.
It is also known from Cuk et al. U.S. Pat. No. 4,184,197 dated Jan. 15, 1980 and entitled xe2x80x9cDC-To-DC Switching Converterxe2x80x9d to provide a so-called Cuk converter. The Cuk converter comprises a boost regulator and a buck regulator coupled in cascade, in which a series circuit comprising two inductors and a series capacitor between them is coupled between an input terminal and an inverted polarity output terminal. In an isolated form of the Cuk converter, the series capacitor is replaced by a transformer and two capacitors, the capacitors being in series with the primary and secondary windings of the transformer.
Despite many improvements in DC converters over many years, there remains a need for further improvements in many areas, including such areas as efficiency, simplicity of the converters and their switch control circuitry, versatility for providing for different input and/or output conditions, cost, and size.
According to one aspect of this invention there is provided a DC converter comprising a buck or boost regulator coupled between input terminals for a supply voltage and output terminals for a regulated output of the converter, the regulator including at least a first switch which is controllable for regulating said output, and a transformer, wherein the regulator comprises an input circuit coupled to the input terminals and having a first capacitor connected in series with a primary winding of the transformer, and an output circuit coupled to the output terminals and having a second capacitor connected in series with a secondary winding of the transformer, and wherein successive cycles of operation of the converter each comprise a phase of energy transfer via the transformer from the first capacitor to the second capacitor during a first state of said switch, and a phase of charge balancing between the capacitors during a second state of said switch.
In one embodiment of the invention, said first switch is connected in series with the first capacitor and primary winding between the input terminals, and the input circuit further comprises a second switch in parallel with the series-connected first capacitor and primary winding, the second switch being controlled in a complementary manner to the first switch. In another embodiment, the input circuit comprises four switches connected to the input terminals in a full bridge arrangement, the series-connected first capacitor and primary winding being connected in a diagonal of the bridge. In a further embodiment, the input circuit comprises two switches connected in series, and two capacitors connected in series, between the input terminals, the primary winding being connected between a junction between the two switches and a junction between said two capacitors, said two capacitors constituting said first capacitor connected in series with the primary winding.
In each case the output circuit can comprise an inductor connected in series with the second capacitor and secondary winding between the output terminals, a diode or switch in parallel with the series-connected second capacitor and secondary winding, and a capacitor coupled between the output terminals.
The transformer primary and secondary windings can be electrically isolated from one another to provide an isolated DC converter, or the transformer can be an autotransformer.
Another aspect of the invention provides a DC converter comprising: a transformer having a first winding and a second winding; first, second, and third controllable switches; first, second, and third capacitors; and an inductor; wherein the first winding, first capacitor, and first switch are coupled in series between a first pair of terminals; the second switch is coupled in parallel with the series-connected first winding and first capacitor; the second winding, second capacitor, and inductor are coupled in series between a second pair of terminals; the third switch is coupled in parallel with the series-connected second winding and second capacitor; and the third capacitor is coupled between one of the first and second pairs of terminals.
A further aspect of the invention provides a DC converter comprising a transformer having a first winding coupled in series with a first capacitor and a first controllable switch between a first pair of terminals, and a second winding coupled in series with a second capacitor and an inductor between a second pair of terminals; a second controllable switch coupled in parallel with the series-connected first winding and first capacitor; a diode coupled in parallel with the series-connected second winding and second capacitor; and a third capacitor coupled between one of the first and second pairs of terminals.
In each case the converter may also include a fourth capacitor coupled between the other of the first and second pairs of terminals.
In one embodiment, the first capacitor is constituted by two capacitors each connected between the first winding and a respective terminal of the first pair of terminals. Another embodiment of the converter includes two further controllable switches via which the first winding is connected to respective terminals of the first pair of terminals.
Conveniently, each controllable switch comprises a semiconductor switch having a reverse parallel diode, and the diode coupled in parallel with the series-connected second winding and second capacitor is constituted by a reverse parallel diode of a further controllable semiconductor switch.
A further aspect of the invention provides a converter comprising: a first switch, a first capacitor, and a first winding of a transformer coupled in series between a first pair of terminals; a second switch coupled in parallel with the series-connected first capacitor and first winding; a second winding of the transformer, a second capacitor, and an inductor coupled in series between a second pair of terminals; a unidirectionally conductive device or a third switch coupled in parallel with the series-connected second winding and second capacitor; a third capacitor coupled between the first or the second pair of terminals; and a circuit for controlling the switches, the first and second switches being controlled so that they are not simultaneously conductive.
At least one of the switches can comprise a bidirectional switch. The transformer windings can be electrically isolated from one another to provide an isolated converter, or the transformer can comprise an autotransformer.